Both acrylic and modacrylic fibers fall under HTS 5506300000 and are not separated at the 10-digit level in the US schedule, so the duty rate is the same regardless of which variant is imported. However, modacrylic fibers used in flame-retardant or specialty applications may require additional product documentation to support end-use claims or downstream textile certifications.

To fall under heading 5506, the fiber must have been carded, combed, or otherwise processed specifically to prepare it for spinning into yarn. Fibers that have not undergone these opening and alignment steps — such as crimped tow or loose fiber not yet processed — are classified under different headings in Chapter 55, so the processing stage must be clearly documented in commercial invoices and technical data sheets.